The invention relates to a starting-process controller                having a voltage-controlled oscillator (VCO), a power output stage, and a resonance converter, wherein        the voltage-controlled oscillator (VCO) generates the control signals required for the power output stage,        the resonance converter converts the stepped output voltage from the power output stage into a sinusoidal voltage at its output,        the piezomotor is driven by the sinusoidal voltage from the resonance converter,        the motor current that flows when the piezomotor is driven is measured and compared with the phase of the drive voltage in a phase comparator,        the output signal from the phase comparator is a measure for the phase difference at the time between current and voltage,        a phase-locked loop filter smoothes the phase-difference signal,        the smoothed signal controls the oscillator (VCO).        
Known from DE 199 42 269 is an electronic drive for a piezomotor (e.g. a micropush motor). The piezomotor is connected to a phase-regulated a.c. voltage. During operation, the current drawn by the piezomotor is measured by means of a diode. The phase angle of the current is detected by comparison with the voltage fed to the motor. A peculiarity of the piezomotor is that the current through the motor, and hence the power drawn by it, decreases under load. This contrasts with electromagnetic drive systems where the current increases under load.
This peculiarity of the piezomotor is attributable to a rise in the internal resistances of the system.
Hence, when a piezomotor and its drive system are being designed, it has to be borne in mind that the current, or rather the applied motor voltage, has to be corrected when operating under load and the motor power adjusted to the load in this way.
Another known effect is that, if there is a changing, i.e. variable load, then this will change the resonant frequency of the motor at the same time. This, once again, causes the active power drawn by, and the efficiency of, the motor to decrease. The two effects described reinforce one another such that the motor may possibly come to a halt. At the same time, the phase-regulating system goes to a self-locked state, from which it generally does not recover. An automatic restart is no longer possible. The reason for this tip-over or stalling effect is that the oscillator is taken from the capacitive range of operation through its resonance and into the inductive range, which causes a phase rotation.